Thermally insulating glass panels 1 typically comprise two spaced apart sheets of glass, 2 and 3, enclosing a low pressure space; refer to FIGS. 1, 2 and 3 (not shown to scale). These sheets are interconnected by a peripheral joint of fused solder glass 4 and an array of pillars 5.
The pillars 5 ensure sufficient structural strength to withstand the forces imposed by atmospheric pressure, and maintain the sheets of glass spaced-apart. The pillars 5 comprise a preform 6 made of glass, ceramic, metal or other materials completely coated with a layer of solder glass 7. The preform 6 is usually made from the same material as the glass sheets in order that the thermal expansion coefficients of the preforms, the sheets and the solder glass should match. The purpose of the preform is to maintain the separation of the glass sheets during the fusion operation when the solder glass has little mechanical strength.
A pump-out tube 8 is incorporated into the panel and is used during the construction of the panel to evacuate the space between the glass sheets. The pump-out tube 8 is hermetically sealed by fused solder glass 9 to a hole 10 which passes from an interior face of glass sheet 2 to the bottom of a recess 11 in the exterior face of glass sheet 2. The recess 11 allows the pump-out tube 8 to be melted and sealed leaving a stub which does not protrude beyond the plane of the exterior face of the glass sheet 2.
A chemical getter 12 is often included within a machined recess 13 in one of the sheets of glass in the panel to counteract any rise in pressure due to outgassing from the glass.
A low emittance coating may be provided on the interior surface of one or both sheets of glass. The emittance of the coatings is usually between 0.05 to 0.2 to ensure a thermal conductance due to radiation of approximately 0.15 to 0.6 Wm.sup.-2 K.sup.-1 or less.
The internal pressure of the panel is usually below 10.sup.-2 torr and sometimes below 10.sup.-3 torr.